Systems for harvesting solar irradiance, of the type with which the present invention is concerned, have application in remote areas where electricity or other utilities are not readily available. However, persons skilled in the art will readily appreciate that the present invention is more broadly directed to a solar energy conversion system, whether the useful energy is in the form of electricity or heat, and irrespective of its ultimate use. Even though the invention has such broader application, it will be disclosed in the context of a source of electrical power and as a source of thermal power for heating and air conditioning, both of which sources are useful in dwellings and office buildings.
In the past, the most widely employed solar energy converters for solar power harvesting have employed a number of photovoltaic cells mounted to a fixed, planar frame; this is sometimes referred to as a “flat panel” or “one sun” construction. The flat panel was positioned in a well-known manner to enhance the collection of useful solar energy. It is known that if solar energy falls perpendicularly onto the surface of a solar conversion cell, the energy conversion is at a maximum. The attitude and elevation of a solar flat panel in a fixed position for a given location on earth will provide a known maximum conversion of solar energy over the solar day throughout the year—that is, the number of generated watt-hours per day.
However, the number of photovoltaic cells required on a fixed flat panel for a usable power station, considering the various positions of the sun throughout the year, is so large that the system has been prohibitively expensive for conventional commercial use. Performance of this flat panel has been enhanced by providing a motor drive to point the panel at the sun through its diurnal travel. Enhancing the energy harvesting of one-sun collectors was accomplished by mounting the cell array on a tracking device. However, this required the use of a heavy frame and support structures to provide adequate wind resistance. Typically expensive mounting or base structures were required with tracking structures. This further increased the cost of fabricating, installing and maintaining such systems. Exposure to the environment resulted in corrosion, the most frequent cause of system failure.
This has been further enhanced by development of successive generations of more-efficient solar cells: at present typically 15%-40% conversion of sunlight into electricity is possible.
An important improvement has been made to decrease the cost of the photovoltaic material by concentrating the sunlight, by a rotationally symmetric parabolic reflector or a linear parabolic trough, which is driven to track the direction of the sun. Absent a use remote from a conventional power source, or else absent government subsidies, a tracking solar concentrator now offers the only potentially cost-effective approach to solar power generation.
Numerous methods for tracking the sun with a single-aperture concentrator such as a parabolic dish or trough have been taught; numerous others have addressed the use of co-tracking or group-deformable subapertures. Generally these involve the use of mechanical linkages, gears, and chain and sprocket drives; aside from general failure due to corrosion and wear, at best these generally are subject to pointing inaccuracy—and thus to loss of concentration effectiveness—through wind pressures, gear backlash, and pointing resolution.
In a perfect concentrator, the image of the sun will be imaged exactly into a minimal photovoltaic cell array. The angular size of the sun is ½ degree: thus if such a concentrator has a tracking error of ½ degree, it will capture none of the direct solar energy.
In all cases the commercial practicality of these successive innovations has been critically limited by the adverse disparity between the value of the solar-generated electrical power as compared with the amortized aggregated cost of the concentrator structure, the drive apparatus, and the photovoltaic cells themselves. Typically the time for return of the investment has exceeded the projected life of the apparatus.
In part because of the cost of the cells, others have sought to convert the sunlight directly into heating a working fluid. Typically solar hot water is used to provide domestic hot water; this can be obtained by circulating water through a simple rooftop array of black absorbing tubes. Today solar heating can further be utilized to power an absorption air conditioning or refrigeration unit utilizing a condensable fluid: specifically, for the “drying” or desorption portion of the cooling cycle. Less frequently, a solar concentrator is used to vaporize a working fluid to operate a generator as by a Rankine or Stirling cycle engine. In any of these cases the typical purely economic net benefit of investment has been approximately zero by the end of the useful life of the equipment.
Thus, an important aspect of a solar power station is its cost effectiveness: that is, the consideration of the total costs of acquisition, delivery, installation, maintenance, fuel, life expectancy, and the like—versus the market value of the utilities it would replace.
In a solar energy conversion system, the costs may be divided into three general areas. First, there is the necessary quantity of solar photovoltaic cells needed to provide the desired watt-hours of electrical energy per unit of time (usually the average minimum number of hours of sunshine per day). Secondly, there is the cost of electrical or mechanical parts in the system other than the solar cells, and the fabrication and installation costs. Finally, to be practical the life expectancy of a solar energy system should generally be at least 22 years, and therefore, maintenance and repair costs must be considered as part of the initial design. The past use of shafts, bearings, mechanical linkages, gears, and chain and sprocket drives to achieve solar tracking has militated against a cost-effective system maintenance cost and useful life. A common failure of prior systems has been due to physical damage and corrosive effects of exposure to the natural elements of wind, rain, snow, hail, humidity, dust, etc. Prior methods of minimizing the effects of weather have proved either too costly or too ineffective for sustained commercial use.
Electrical solar power has a value related to the conversion efficiency of the photovoltaic (PV) panel and to the cost of the net electricity it replaces; the production value of solar thermal power is related to the relatively low cost of the fuel it replaces. Almost all prior art has taught the production of only one and not the other. Some have taught the production of both electricity and useful heat, but with problems of efficiency and/or construction. While U.S. Pat. No. 7,173,179 teaches the use of a spectrum-splitting chemical solution which absorbs infrared light and transmits visible light to silicon on the rearward surface, this design significantly compromises the potential performance of the costly silicon. U.S. Pat. No. 5,522,944 teaches the use of a fixed plate of heat-absorbing fluid pipes in between solar cells, but without harvesting both electricity and heat at any one point of irradiance. U.S. Pat. No. 5,505,788 teaches a fixed roofing system in which convecting water cools a solar cell array; aside from this requiring that the entire roof be plumbed as a single water vessel, no provision is made for solar concentration.
It is well known that a major component of the entire system cost for a solar collector is the cost of installing the collector. In the case of systems that utilize a tracking device there is a major cost in both the tracker and the structural base for the tracker that is required for wind loading on the collectors that are mounted to the tracker and act as a sail.
Solar energy harvesters heretofore known suffer from a number of disadvantages:
(a) They utilize shafts, bearings, and pinion gears to enable rotation, with adverse effects on purchase cost, lifetime, maintenance cost and tracking accuracy.
(b) They utilize rigid parabolic concentrators of a mirror nature, which are expensive in manufacture, and structurally three-dimensional: with significant vertical dimension.
(c) They utilize tracking means which are necessarily massive to overcome wind pressures, costly to purchase, and require periodic and costly maintenance over a 22 year expected lifetime.
(d) Typically the time for return of the investment has exceeded the projected life of the prior art apparatus: the return on investment is negative. Even under the optimistic models, if the payback time exceeds 12 years than it is irrational in an economic sense to invest in solar power rather than in other available, higher-return (>6%) investments; no solar power harvesting system in the prior art has offered such a payback time.
When all the actual costs are accounted, typically the time to return the investment from the value of utilities presently saved (e.g., for San Francisco) ranges from 30 years for a “one-sun” photovoltaic roof-cover to between 30 and 150 years for a state of the art two-axis tracking parabolic dish concentrator.
(e) Substantially all the prior art has taught the harvesting of high value electrical power from the sunlight and considered the other 80% of the absorbed sunlight to be an engineering challenge to dissipate, or else has optimized the harvester to capture 100% as relatively low value heat, sometimes to be converted into electrical power through the further losses of conventional heat-engine generators.
(f) The optimum orientation of prior solar power harvesters, whether sun-tracking diurnally or not, is to be tilted toward the south at the angle of the local latitude. This requires a fortuitous choice of roof or else a construction of significant complexity and skyline bulk. Even with the most costly tracking: two axis, including the ±23.26° as the sun moves through its seasons, can collect a maximum of one sun over its area.